1. Field of the Invention
The present invention relates to a pneumatic tire, more particularly to a bead structure adapted for a 15-degree-taper rim.
2. Description of Related Art
15-degree-taper center-drop rims are widely used as suitable for pneumatic tires for heavy duty use such as for trucks, buses and the like because a large engaging force is produced between the tire beads and rim bead seats as the tire is inflated.
In such tires, although the rim bead seat is provided with a single taper of 15 degrees, the tire bead base is, as shown in FIG. 3 by a dashed line, often provided with a double taper of about 21 degrees and 31 degrees (parts b1 and b2) in order to improve contact between the rim bead seat and the tire bead base which provides airtightness necessary for inflating the tire during mounting operation (initial inflation).
In case of this double tapered bead base (b1 and b2), as the inside diameter at the bead toe (P2) becomes small, the bead toe gets stuck with a rim flange and tends to chip off when the bead portion goes over the rim flange during mounting the tire on the rim. On the other hand, in order to obtain a large engaging force, it is necessary for the rubber beneath the bead core to relatively decrease its thickness. This is however, liable to cause cracks in this portion during long-term use.
It is therefore, an object of the present invention to provide a pneumatic tire in which bead damage such as bead toe chip-off and cracks can be prevented without sacrificing airtightness and rim mounting operation.
According to the present invention, a pneumatic tire to be mounted on a 15-degree-taper rim, comprises a tread portion, a pair of sidewall portions, a pair of bead portions each with a bead core therein, and a carcass comprising a ply of steel cords extending between the bead portions through the tread portion and sidewall portions, wherein
in a meridian section of the tire under a state that the tire is not mounted on a rim but the bead portions are held such that the bead width is equal to the rim width of the 15-degree-taper rim,
the bead base surface of each bead portion, which is defined as extending between a bead heel and a bead toe, is substantially straight and inclined at an angle of from 22 to 26 degrees with respect to the tire axial direction, and
a ratio La/Lb of the axial width La of the bead base surface to the maximum width Lb of the bead core is in a range of from 1.3 to 1.6